Oceanography Chapter 9 10% water in the world is tied up in the surface ocean currents. (above the pycnocline) ¾ Primary source is wind: Westerlies, Trades, Polar Easterlies Coriolis deflects winds (and water) and produces the gyres. >>Gyres-deflection of water into a circular pattern North Atlantic Gyre: Gulf Stream ⇒ North A. Current ⇒ Canary current ⇒ North EQ. Current Water of depth moves of an angle to the right of the overlying water ⇒ Ekman Spiral (Fig 9.5) ¾ Due to friction mostly Ekman Transport – the sum of all the arrows indicating water direction in the affected layers ¾ In theory, Ekman Transport is 90° to the right of the wind direction (N. Hemisphere) Reality ≈ 45° ¾ Produces hill of water in Sargasso Sea area. Geostrophic Gyres- balance between, PGF and Corolis (Earth Turning) ¾ 2 in Northern Hemisphere ¾ 3 in Southern Hemisphere ¾ WWD (Antarctic Circumpolar Current) Classified according to their location in the Gyre: 1. Western Boundary Currents-warm ¾ Gulf Stream, Kuroshio ⇓ Westerly Wind driven 55 million m3/s or 55 sv ¾ Form Eddies (turbulent rings) 1. Cold core –rotate counter clockwise Table 9.1 2. Warm core – rotate clockwise 2. Eastern Boundary Currents – cold ¾ CA, Benguela, Peru, West Australian Small 10-15 sv 3. Transverse currents ¾ Flow E ⇒W or W ⇒E ¾ Tradewind driven: N. Equatorial, S. Equatorial Atlantic – 8inches higher ¾ 30 sv westward Pacific – 1m (3.3 feet) Currents generated by the Tradewinds are more narrow and faster then the westerly driven transverse currents of the mid -latitudes. The Northern Hemisphere transverse currents are interrupted by continents. Southern Hemisphere – West Wind Drift – carries 100 sv, move than any other current. (up to 10x) Westward Intensification: Western boundary currents are faster, deeper, and narrower (up to 20x) then eastern boundary currents. ¾ Coriolis causes it to make the turn fast and spread out. (Figure 9.13) Countercurrents and Undercurrents Countercurrents- Flow on the surface in the opposite direction from the main current Undercurrents- Countercurrents beneath surface currents. ¾ Cromwell upwells at Galapagos ¾ Coriolis keeps this current in line Exceptional Surface Currents 1. Monsoon currents – summer, ITCZ moves north, and the winds blow from the south and the southwest – reverses Gyre to flow clockwise (creates Eastward moving Southwest Monsoon Current, instead of the N.E.C) 2. High Latitude Currents ¾ Form when larger currents are split and deflected by collision with a continent. Labrador and Greenland currents transport nutrients to the rich fishing area of New Foundland. Effects of Surface Currents on Climate Heat Exchange - Transfer occurs at the mid –latitudes Warm water currents make Northern cities more maritime. Example: Cold water currents make rainfall totals lower. Example: Mark Twain – “Coldest winter I ever spent was a summer in S.F.” (compare with similar latitudes on East Coast) DC-cold in winter (Westerly influence), humid – hot in summer Upwelling and Downwelling (wind – induced vertical circulation) Wind driven horizontal movement of water can sometimes induce vertical movement in surface water. 1. Equatorial Upwelling – water moving in the currents on either side of the equator is deflected slightly poleward and replaced by deeper water (Fig 9.15) ¾ Makes waters nutrient rich-more biological activity ¾ Layers of ooze on the Pacific equatorial seabed) 2. Coastal Upwelling – wind blows parallel to the shore, Coriolis effect deflects water to the right, and the resultant Ekman Transport (direction of Ekman Transport is 90° to the right of the wind direction – offshore) moves the water offshore. This water is replaced by rising water from below. ¾ Keeps the water cold, but nutrient rich (fig 9.16) 3. Downwelling – water driven toward a coastline will be forced downward, returning seaward along the continental shelf, helps deeper critters. Langmuir Circulation – wind that blow steady across the ocean can induce long sets of counter – rotational cells in surface water. Fig. 9.18 ¾ Windows – areas of convergence ENSO (El Nino Southern Oscillation)-Fig 9.19 Trade winds diminish and than reverse, due to a High Pressure building in the western Pacific. Results : 1. Sea level rise – 8 inches 2. More evaporation (more rainfall) El Nino – shows up at Christmas Normal circulation returns with vigor – La Nina ¾ Stronger upwelling 1982-1983 El Nino – La Chichon Thermohaline Circulation- movement of water due to differences in density Water masses do no easily mix (just like air masses) 5 Common Water Masses: 1. Surface water: to a depth of 200 m (600 ft) 2. Central water: to the bottom of the main thermocline (varies with latitude) 3. Intermediate Water: to about 1500 m (5000 feet) 4. Deep water: Below IW, but not in contact with the bottom, to about 4000m (13k feet) 5. Bottom water: water in contact with the bottom Ocean is density stratified. Visualize ocean layering – Temperature - Salinity diagram - figure 9.21 ¾ Many combination of T&S yield the same density ABW-Antarctic Bottom Water ¾ 34.65‰ salinity ¾ 30°F (-0.5° C) ¾ 1.0279 gl cc Produced along Antarctic coasts, forms in the Waddell Sea Flows North: Pacific = 1000 yrs to reach EQ, 600 m to reach 50° Atlantic: 750 years to reach 40°N NADW- North Atlantic Deep water ¾ very little Arctic ocean water gets into the Pacific ¾ water at Iceland’s Latitude cools from 10°-2°C (50-36°C) and sinks Pacific Deep water (PDW) ¾ forms along WWD and Kamchatka Peninsula Both PDW and NADW are less dense than ABW, so they float above it, out of contact with the ocean bottom. MDW – Mediterranean Deep water ¾ lots of evaporation (300 k km3, 72 km3) ¾ Produces very saline (38 ‰) water which flows into the Atlantic ¾ Sits on top of others because it is warmer, and therefore less dense Researchers can determine the age of deep water by analyzing its DO (dissolved oxygen) content. ¾ Mix more and lose identity with age. Figure 9.24 – classic model of thermohaline circulation Convergence Zones - just like tectonics, the most dense sinks Sometimes, you can have the same density with different temperatures. They will mix and produce a denser new water (caballing) and sink. (Look at Fig. 9.21) Thermohaline Time: 1600 yrs ABW 200-300 yrs to rise Gyre – 1 year to spin Contour Currents – faster currents – below the Western boundary currents Convergence Zones – figure 9.26 ¾ The most pronounced zone is the Antarctic Convergence Distribution of heat of tropical waters 3D diagram of figure 9.27 Measuring and Studying Currents 1. Float methods ¾ Drift bottle 30k Nikes 29k Rubber duckies 2. Flow method – flows past a fixed object Cool Gizmos Drogue – drifts with current Rubber Duckie Sofar Float (3500 m) Ekman Flow Meter – current speed and direction Transducer for ADLP System (Acoustic Doppler Current Profiles) Slocum Glider – probe that uses energy from gravity buoyancy, heat and batteries to power long range exploration of water masses .